Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G09: Bubbles: Growth, Heat Transfer and Boiling II |
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Chair: Alexander Yarin, University of Illinois, Chicago Room: Georgia World Congress Center B214 |
Monday, November 19, 2018 10:35AM - 10:48AM |
G09.00001: Boiling on a Wire Heater: Exponential Vaporization Fronts and Vibrations Christopher Staszel, Alexander L Yarin Vapor bubble nucleation on thin strip heaters is explored theoretically and experimentally. Vibrations of the heating surface due to nucleate boiling is also studied theoretically and experimentally. The fundamental phenomena resulting in subcooled boiling-induced vibration (SBIV) is demonstrated to be intrinsically related to the entrained liquid circulation driven by vapor bubble nucleation. In the case of high heat fluxes at the heater, vaporization fronts occur along the thin strip heaters leading to a sudden and violent transition to film boiling. The resulting vaporization front propagation along the heater was studied experimentally and shown to exponentially increase in time, contrary to the prior views of linear propagation. A theoretical description of this new phenomenon, the vaporization fronts, is given and confirmed experimentally. The theory has also provided an explanation and description of the critical heat flux (CHF) and film boiling inception. |
Monday, November 19, 2018 10:48AM - 11:01AM |
G09.00002: Onset of Nucleate Boiling on Soft Surfaces: Theoretical Considerations Akshay Phadnis, Konrad Rykaczewski Surface engineering is often used to alter interfacial phenomena with the aim of reducing superheat required for onset of nucleate boiling (ONB), which can increase efficiency in many industrial applications. Commercially available enhanced boiling surfaces that reduce superheat consist of either porous metal coatings or sub-millimeter metal fins with re-entrant geometries.In the last decade, the majority of the research in this area has focused on boiling enhancement using coatings with nano-to-microscale textures and/or chemical heterogeneities. In this presentation, we will expands beyond these two design variables (texture and chemistry) and provides theoretical arguments highlighting that the superheat required for ONB can be further tuned by modifying the mechanical properties of the boiler surface. Specifically, two bubble formation mechanisms will be evaluated theoretically to determine the effect of surface softening on the superheat required for ONB. First, heterogeneous vapor bubble nucleation on smooth, soft surfaces is mapped in terms of the kinetic limit of the superheat. Second, augmentation of vapor trapping in surface cavities and superheat required to seed bubbles from the ensuing gas pockets induced by softening of the surface is evaluated. |
Monday, November 19, 2018 11:01AM - 11:14AM |
G09.00003: Bubble Dynamics in Sub-cooled Nucleate Boiling Vinod Pandey, Gautam Biswas, Amaresh Dalal Generation, growth and departure dynamics of bubbles are the important physics associated with the study of nucleate boiling. Contrary to the saturated liquid condition, the growth and departure processes under sub-cooled condition have been found to reveal varying dynamical and thermal responses. The phenomenon is dominated by the phase-change occurring as a result of condensation at the interface near the sub-cooled liquid, evaporation at the superheated liquid layer and the microlayer evaporation. The microlayer is a thin liquid layer underneath the growing bubble which is formed as a result of viscous stresses acting at the liquid attached to the wall. The bubble, influenced by the combined effect of various phase-change processes may depart, oscillate or remain attached to the substrate depending on the degree of subcooling of liquid. The present work demonstrates such phenomena exhibited by a single nucleating bubble through numerical simulations using the solutions of complete Navier-Stokes and energy equations. The numerical technique incorporates the combined level-set and volume of fluid (CLSVOF) approach for interface capturing and a microlayer-model to determine the phase-change contribution associated with the microlayer. |
Monday, November 19, 2018 11:14AM - 11:27AM |
G09.00004: An approximate analytical solution for bubble departure diameter in a vertical boiling channel Swapan Paruya, Jyoti Bhati, Subramaniam Pushpavanam This simplified analytical solution for bubble departure diameter was derived in the connection to the need of determining bubble inclination angle, which the existing literature lacks. A large number of correlations for the departure diameter are currently in use for large-scale numerical simulation of boiling flows in channels. However, while solving for the bubble departure diameter numerically, we found that the choice of a proper bubble inclination angle was very crucial. The solution is based on the force balance that considers the major contribution of buoyancy force and growth force. Of course, the surface tension plays a little role at the time of bubble departure. The growth kinetics we used are our own asymptotic solution and those from the literature. The analytical solutions for the departure diameter and the growth time have been compared with our numerical solutions, the existing correlations and our experiments for different superheats (Jakob number Ja) and pressures. Our analytical solution is found to be reasonably accurate in high Ja. An appreciable deviation from the numerical simulation and some experiments in low Ja suggests that the contribution of micro-layer evaporation is important. |
Monday, November 19, 2018 11:27AM - 11:40AM |
G09.00005: Abstract Withdrawn
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Monday, November 19, 2018 11:40AM - 11:53AM |
G09.00006: Numerical investigations on the origin of rapid evaporation Arash Asadollahi, Asghar Esmaeeli, Robert Ferris, James Carl Hermanson The unstable growth of a vapor bubble in a pool of highly superheated liquid has been a problem of long-standing interest. While the origin of the unstable growth has been suggested to be due to Landau instability, Rayleigh-Taylor (spherical) instability, and other instability mechanisms (such as vapor recoil), these speculations have not been fully vetted because of insufficient experimental or computational data. Here we use Direct Numerical Simulations to study the origin of unstable vapor bubble growth for several fluids of interest and compare the results with the theoretical predictions. To this end, we follow the evolution of an initially small vapor seed in a pool of superheated liquid. The surface of the vapor seed is perturbed slightly to trigger instability. The onset of instability is identified when the bubble surface starts to roughen. The effects of the key controlling parameters, such as liquid superheat and liquid/vapor properties on the onset of the instability are investigated. Selective experiments are also performed by University of Washington team to aid the computational modeling. |
Monday, November 19, 2018 11:53AM - 12:06PM |
G09.00007: Direct numerical simulation of gravity effects on pool boiling heat transfer Akash Dhruv, Elias Balaras, Amir Riaz, Jungho Kim Design of efficient thermal systems for satellites and spacecrafts that operate in varying environments require an understanding of how pool boiling scales with gravity. Experimental investigations have identified existence of two distinct boiling regimes dominated by buoyancy (BDB) and surface tension (SDB). Quantitative assessment shows a mathematical relationship between wall heat transfer and gravity as a function of heater size, superheat and degree of subcooling. However, a qualitative understanding of how these parameters affect bubble dynamics is still elusive. High fidelity simulations can reveal useful information about these processes which can be useful in improving the mathematical model. This talk will discuss a numerical framework to simulate pool boiling in low gravity conditions, and show the effects of gravity and subcooling through three-dimensional calculations. Focus will be given towards explanation of dynamics in transition gravity levels between BDB and SDB regimes. Results will be verified with experiments and computational challenges associated with resolving the thin liquid thermal boundary layer will be addressed. A statistical description of heat transfer, near wall flow and bubble dynamics will also be presented. |
Monday, November 19, 2018 12:06PM - 12:19PM |
G09.00008: Simultaneous spatiotemporally resolved temperature and velocity field measurements of nucleate boiling Victor Voulgaropoulos, Gustavo Matana Aguiar, Artyom Kossolapov, Bren Phillips, Omar K Matar, Matteo Bucci, Christos N Markides Boiling is exploited in applications where high heat transfer rates are required, however, our fundamental understanding of the complex underlying thermo-fluid physical processes and their interactions across scales that give rise to this phenomenon remains incomplete. Experiments of nucleate boiling are performed in which the temperature of the heated surface is recorded with an infrared camera simultaneously with the application of laser-based techniques that provide space/time-resolved information on the temperature and velocity fields in the liquid surrounding a bubble during its nucleation, growth and detachment. A laser excites two fluorescent dyes and microparticles introduced into the flow as markers. The fluorescent signals of the dyes are captured individually by two high-speed cameras, and their ratio is used to measure the temperature around the bubble and to identify the corresponding thermal boundary layer, while limiting the effect of interface reflections. At the same time, particle image/tracking velocimetry algorithms are applied to isolated tracer particle images in order to reveal the flow field around the bubbles. |
Monday, November 19, 2018 12:19PM - 12:32PM |
G09.00009: Pool Boiling: Effect of pool depth, polymer nano-textured heater surface and self-rewetting fluids Abhilash Sankaran, Wenshuo Zhang, Alexander Yarin Pool boiling of ethanol and self-rewetting fluids were studied on bare copper surface and surface with polymer nanofibers. Experiments with different pool depths of ethanol revealed insignificant differences in heat removal rate indicating that the heat transfer in the liquid microlayer is the important factor. Also, several alcohol solutions at 0.1% (v/v) concentration including self-rewetting fluids were investigated as working fluids. It was found that only the high carbon-alcohol improved boiling performance considerably at 0.1% (v/v) concentration. Further, boiling performances on polymer nano-textured copper surface were compared to the cases with bare copper heater. Two different polymer [polyacrylonitrile (PAN) and polystyrene (PS)] nanofibers were deposited on the copper surface. Measurements of boiling curve revealed a detrimental effect of PAN nanofibers in the case of ethanol and self-rewetting n-heptanol solutions. However, PS nanofibers revealed significant improvement in boiling heat transfer with n-heptanol and water. The reason for this contrast is attributed to the increased wettability of the corresponding working fluids on PAN and the decreased one on PS fibers compared to the bare surface. |
Monday, November 19, 2018 12:32PM - 12:45PM |
G09.00010: Vaporization and bubble growth dynamics of explosively boiling droplets at the superheat limit F. Robert Ferris, James C. Hermanson, Arash Asadollahi, Asghar Esmaeeli The explosive boiling of droplets of diethyl ether (0.5-4.0 mm in diameter) at the superheat limit was examined experimentally. Droplet explosion was studied using a heated host fluid column to bring test droplets to the superheat limit. The droplet fluid was diethyl ether (superheat limit 147 C at 1 bar) with glycerol employed as the host fluid. Imaging of the interfacial behavior during explosive boiling was performed using a high-speed camera at a frame rate of 100,000 frames per second. Tests were carried out at pressures between 1-5 bar absolute. At 1-2 bar the relatively rough vapor-liquid interface inside the droplet suggests unstable boiling. Stable boiling was encountered between 4-5 bar; at 3 bar a transition from stable to unstable boiling was sometimes observed. The speed of the vaporization front within the droplet rises rapidly after nucleation before decaying. The mean wavelength at the vapor-host liquid interface appears in all cases to show a modest variation with each subsequent vapor bubble oscillation. Direct Numerical Simulations were performed at Southern Illinois University Carbondale to compliment the experiments. |
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